A conventional turbo-molecular stage arrangement of a vacuum pump comprises a stack of alternate rotors and stators. Each stage effectively comprises a solid disc with a plurality of blades depending (nominally) radially therefrom; the blades are evenly spaced around the circumference of the disc and angled "about" radial lines out of the plane of the disc in the direction of rotation of the rotor stage.
The rotor and stator blades have positive and negative gradients respectively when viewed from the side in a radial line from the disc. This arrangement has the effect in molecular flow conditions of causing the movement of molecules through the pump.
There are a number of types of apparatus where a plurality of chambers needs to be evacuated down to different levels of vacuum. For example, in well known types of mass spectrometer that part of the apparatus known as the detector commonly has to be operated at, say 10.sup.-6 mbar whereas that part known as the analyser has to be operated at a different level of vacuum, say 10.sup.-3 mbar.
In addition and importantly, the throughput of gas from the different parts of the apparatus will generally vary also. For example in a typical mass spectrometer of the type discussed above, there may need to be a 60 l/second capacity for the detector and a 200 l/second capacity for the analyser.
In apparatus of the type including but not restricted to mass spectrometers, a number of different vacuum pumps are normally employed. For example, in mass spectrometers, the detector and analyser may be evacuated by separate turbo-molecular vacuum pumps which themselves need to be backed by separate pumps, for example rotary vane pumps.
There is an ever increasing need to rationalise the use of the various vacuum pumps for overall reduced apparatus size and power requirements. A single backing pump is relatively common for supporting two (or more) turbo-molecular pumps. In addition, it has more recently been proposed to employ a single turbo-molecular pump to replace two (or more) individual pumps with the single pump having a normal inlet for gas required to pass through all the stages of the pump and an intermediate inlet, i.e. between the stages, for gas required to pass through only the latter stages of the pump.
However, even these proposals for rationalisation of the apparatus pumping system do not overcome all the problems associated with size and power consumption in particular.
There is therefore a need for improved vacuum pumps in which rationalisation can be further enhanced.